Place one 24-inch 2-by-4 on a flat surface with the four-inch side facing down. Place a piece of 42-inch 2-by-4 on one end and a piece of 39 1/4-inch 2-by-4 on the other so they are perpendicular to the 24-inch piece. Attach these pieces together by driving two screws in a horizontal line driven through the 24-inch piece into the ends of the perpendicular pieces.
Repeat Step 1 to attach the other 24-inch, 42-inch, and 39-3/4-inch pieces together. These are the sides of the stand.
Place a piece of 8-inch 2-by-4 on top of both 42-inch pieces so it is parallel to the 24-inch pieces and one edge of the wood is in line with the edge of the 42-inch piece. Attach the 8-inch pieces to the top of the 42-inch pieces by driving two screws in a line through the 8-inch pieces into the tops of the 42-inch pieces.
Place the piece of 36 3/8-inch 2-by-4 at the ends of the 8-inch pieces of 2-by-4 to connect the two sides. Attach this by driving screws through the 36 3/8-inch wood into the 8-inch pieces.
Measure six inches from the top of the 24-inch 2-by-4 up the 42 inch piece and make a mark. Place the piece of 36 3/8-inch by 20-inch plywood inside the stand base at this point. This is a shelf to hold the hard drive of the computer. Screw this to the stand by driving screws through the front and back 2-by-4s and into the edges of the plywood.
Place the piece of 15 3/4-inch by 35 7/8-inch plywood on top of the 39-3/4 inch pieces of 2-by-4 so that one of the longer edges is even with the edges of the 39 3/4-inch 2-by-4. Screw this piece by driving screws through the plywood into the tops of the 2-by-4. This is the shelf for the monitor.
Install the plastic keyboard shelf in front of the 39-3/4-inch piece and in between the 42-inch pieces of 2-by-4. Follow the directions you get with the shelving kit to ensure a good fit.